1. Field of the Invention
The present invention relates generally to circuits for monitoring current. More particularly, the present invention relates to a monolithic integrated circuit that can be used to detect faulty lamps in automobile electrical systems.
2. Discussion of the Related Art
In electrical systems, it is often desirable to monitor the current consumed by various loads to determine whether a particular load is functioning properly.
In automobile lighting systems in particular, it is often desirable to monitor the condition of various electrical loads within the automobile such as motors, external vehicle lamps such as signal and warning lamps, and defoggers to determine whether or not a particular load is functional. It is also desirable to be able to test the fuses which supply various circuits in the automobile to determine whether or not the fuses are functioning properly. Within this disclosure, monitoring current through lamps is discussed merely as an illustrative example of the broader concept of monitoring current through electrical loads. The purpose of monitoring the condition of the various electrical loads such as signal and warning lamps in the automobile is to alert the driver to a failure of any one of these loads. Obviously, the failure of a warning or a signal lamp in the automobile poses a safety hazard.
Current monitoring in an automobile lighting circuit is generally accomplished by inserting a current shunt in the lamp circuit. The current shunt is generally a two-terminal resistive element, having one end connected to the automobile power supply source, which is usually the battery, alternator, and voltage regulator, and the other end connected to the load on the circuit, which is usually one or more incandescent lamps or other electrical load. Since the full circuit current passes through the resistive shunt, the resistance of the shunt should be as low as reasonably practicable to avoid unnecessary power consumption as well as excessive heating of the shunt and wiring. The current shunts can be discrete units constructed of special materials, or can be portions of the wiring which supplies power to the lamp being monitored, such as printed circuit board traces or wires in the automobile wiring harness.
Since, as stated, the resistance of the shunt is generally very low, the voltage drop across the shunt which occurs when a particular automobile lamp is activated is correspondingly small. For example, if the battery voltage is 12.000 volts at the input to the shunt, the voltage at the terminal of the shunt connected to the lamp may be on the order of 11.997 volts; the resulting voltage drop across the shunt being approximately 3 mv. Thus, the current monitoring circuit must be able to detect a very small threshold switching voltage which is offset by the much larger power supply voltage.
An integrated circuit capable of using a printed circuit trace as the current shunt and detecting threshold switching voltages on the order of a few millivolts can be seen in my copending patent application Ser. No. 07/410,988, U.S. Pat. No. 5,095,274 entitled Temperature Compensated Apparatus for Monitoring Current Having Controlled Sensitivity to Supply Voltage.
Although that circuit works adequately and is able to use a copper printed circuit board trace as the current shunt in a lamp monitoring circuit, the circuit does have limitations. First, the circuit is designed to track changes in the resistance of a current shunt due to changes in temperature of the shunt where the shunt is a copper printed circuit board trace.
Second, the circuit has a fixed controlled sensitivity to changes in the circuit supply voltage. It is known that the current in an incandescent lamp typically varies at approximately one half the rate of variation of the voltage applied to the lamp. For example, if the voltage applied to the lamp doubles, the lamp current does not double, but rather increases by 50%. Thus, the circuit has to be able to track this variation in lamp current as the circuit supply voltage varies in order to adjust a threshold switching voltage to track this change in lamp current. However, the circuit of application Ser. No. 07/410,988, U.S. Pat. No. 5,095,274 is limited to a 50% sensitivity to changes in circuit supply voltage. If a different lamp were used having a different relationship between applied voltage and current consumption or if a different characteristic sensitivity to changes in supply voltage is desired or required, the previous circuit may not be easily adapted to compensate for this change.